1. Introduction
In my studies on the electrodeposition of aluminum, I have discovered that the conventional methods of cleaning the substrate before plating, such as ultrasonic cleaning, the use of various alkaline cleaners, the use of various acid cleaners, and electropolishing in aqueous baths, followed by an aqueous rinse cycle and dry cycle yield adequate deposits of adherent aluminum onto certain steel alloys, or certain noble metals. However, if the substrate is titanium, tantalum or certain nickeliron alloys such as Kovar and Alloy 42, the application of the above techniques results in deposits of aluminum that have only low adherence to the substrates. Hence, there is a need for a novel etch solution that would activate these and other metal surfaces such that once these surfaces are etched, electrodeposition of adherent aluminum may proceed immediately - without exposing these etched surfaces to air or aqueous media by going through a rinse cycle just prior to plating.
2. Prior Art
Anodic etching techniques or reverse current electropolishing methodsare, broadly speaking, not new (1, 2, 3, 4, 5)*. The most commonly used electropolishing baths are, however, based upon the use of sulfuric and phosphoric acids and these electropolishing baths are found not to be effective in the electrodeposition of aluminum onto the substrates above mentioned. The major disadvantage of utilizing these baths in the precleaning of the particular metallic substrates listed above for aluminum plating is that immediately after the metallic substrate is etched, an additional aqueous rinse step and a drying step are necessary before plating can proceed. These rinse and dry steps are necessary not only to remove traces of etch solution adhering to the surface of the substrate in question, but also to prevent the sensitive electroplating solution (which is preferably non-aqueous) from being contaminated with the aqueous etch solution. Thus, an oxide layer usually re-appears on the supposedly "cleaned" surface of the substrate, which gives non-adherent deposits of aluminum. So far as we are aware, no prior art has been developed or reported involving the use of organic etch baths, or other electropolishing solutions, that will produce adherent aluminum deposits upon plating onto the particular metal substrates cited above. FNT *(1) C. L. Faust, J. Electrochem, Soc. 75, 62c (1949) (2) C. L. Faust, Proc. Am Electroplat, Soc. 137 (1950) (3) C. L. Faust, Met. Prog. 71(5), 101 (1957) (4) C. L. Faust, "Metal Finishing Guidebook - Directory" Metals and Plastics Publication, Westwood, N. J. (Annual) (5) A. K. Graham, Ed., "Electroplating Engineering Handbook" 3rd Ed., Van Nostrand Reinhold, Princeton N. J. (1971)